In a method for producing welded helical seam tubes with optimized tube geometry a metal strip is formed to an open seam tube by means of a forming unit and the converging band edges are welded together and wherein an actual diameter which is measured on the welded tube is compared to a predetermined target diameter and a possible deviation which lies outside a tolerance threshold is used for correction of the actual diameter, wherein the complete welding of the band edges takes place in two steps with a tack welding as first step immediately after the forming as open seam tube and a subsequent final welding as second step, wherein the measurement of the tube diameter takes place after the complete forming to an open seam tube and before the final welding. A device for carrying out the method includes a forming unit in which a metal strip is formed to an open seam tube and a welding unit for tacking the tube, a welding station for final welding of the tube and a device for measuring the diameter D of the tube, wherein the diameter measuring device is arranged so that either the completely formed open seam tube or the only tack welded tube is measured.
For the transport of water, oil and gas submerged arc welded helical seam tubes, also referred to as spiral tubes are normally used, which are preferably produced from steel strips or from steel sheet.
From the brochure “spirally welded large tubes—product information—(Salzgitter Mannesmann large tube 3/08) it is known to form the strip helically into an open seam tube in a forming device and to weld the strip to a tube in a two step process.
For this purpose, the hot strip is formed into a tube in a forming unit of a tube forming machine. The forming unit includes a 3-roller beam bending system with an outer roller support cage and a so called offset roll. With the height adjustable offset roll, a possible strip edge offset of the open seam tube can be compensated.
The tube diameter is influenced by the feed angle of the strip into the forming unit and by the strip width of the used starting material. The diameter of the tube can also be influenced by means of the height adjustable offset roll.
In this manufacturing process known as “HTS-method” the strip edges of the open seam tube are welded by means of inert gas tack welding at high welding speed of up to 15 m/min, wherein the strip edges are only partially joined.
The final welding with a complete welding of the band edges with an inner and outer seam occurs subsequently in a second step on separate welding stations by means of submerged arc welding.
The advantage compared to conventional single step processes, in which the submerged arc welding seams are also produced directly in the tube forming machine and welding of the tube is thus completed in one step is that a higher capacity of the tube forming machine is achieved by the high speed of the tack welding.
Further, the generated tube geometry is not compromised by the UP (submerged arc) welding process, and with this compliance with predefined tolerance values improved.
Ever increasing demands on diameter tolerances to be complied with result in a high expense to accurately determine the tube diameter and to correct the influenced parameters if necessary.
Deviations of the tube diameter from the predetermined tolerances can for example be caused by deviations of the band width and/or the strength and expansion of he used material. These changes affect the forming process and with this the geometry of the tube.
In order to be able to influence the diameter of the welded tube it is known for example from JP 56105816 A and JP 61180613 A to measure the diameter of the completely welded tube and to use the measurement for the correction of the tube diameter. In the known processes, the tube is conventionally welded in a single step process, i.e. welding of the tube into an open seam tube is directly completed after forming of the strip.
For determining the diameter of the finished welded tube it is known for example to use laser triangulation or ultrasound. However, oftentimes the circumference of the tube is still measured with a band measure and the diameter determined there from.
The determined measurements subsequently serve for changing the feed angle of the strip or the exit angle of the welded tube (JP 611806613) or to influence the diameter of the welded tube by adjusting the offset roll (JP 56105816 A).
A disadvantage of all know methods is that the diameter is only measured after completing the welding of the tube and that only then the previously mentioned corrections are possible. The tube, which may be very long at the time of the correction, subsequently has to be reworked with high effort or even be scrapped.